This invention relates to interferometers, e.g., displacement, angle, and dispersion measuring interferometers for measuring the position of a measurement object such as a mask stage or a wafer stage in a lithography scanner or stepper system.
Displacement measuring interferometers monitor changes in the position of a measurement object relative to a reference object based on an optical interference signal. The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object.
In many applications, the measurement and reference beams have orthogonal polarizations and frequencies separated by a heterodyne, split-frequency. The split-frequency can be produced, e.g., by Zeeman splitting, by acousto-optical modulation, or by positioning a birefringent element internal to the laser. A polarizing beam splitter directs the measurement beam along a measurement path contacting a reflective measurement object (e.g., a stage mirror), directs the references beam along a reference path, and thereafter recombines the beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam that passes through a polarizer that mixes polarizations of the exit measurement and reference beams to form a mixed beam. Components of the exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams. A detector measures the time-dependent intensity of the mixed beam and generates an electrical interference signal proportional to that intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a xe2x80x9cheterodynexe2x80x9d signal at the split frequency. When the measurement object is moving, e.g., by translating a reflective stage, the heterodyne signal is at a frequency equal to the split frequency plus a Doppler shift. The Doppler shift equals 2 vp/xcex, where v is the relative velocity of the measurement and reference objects, xcex is the wavelength of the measurement and reference beams, and p is the number of passes to the reference and measurement objects. Changes in the optical path length to the measurement object correspond to changes in the phase of the measured interference signal, with a 2xcfx80 phase change substantially equal to an optical path length change nL of xcex/p, where n is the average refractive index of the medium through which the light beams travel, e.g., air or vacuum, and where L is a round-trip distance change, e.g., the change in distance to and from a stage that includes the measurement object. Similarly, multiple interferometers can be used to measure changes in distance to multiple points on the measurement object, from which changes in the angular orientation of the measurement object can be determined.
For high performance applications such as IC manufacturing the quantity of interest is the geometrical length L and not the optical path length nL, which is what is measured by the displacement measuring interferometer. In particular, changes in nL can be caused by changes in the refractive index n rather than by geometric changes in the relative position of the measurement object. Techniques based on dispersion interferometry have been used to compensate displacement measurements for air turbulence. In particular, interferometric displacement measurements are made at multiple optical wavelengths to determine the dispersion of the gas in the measurement path. The dispersion measurement can be used to convert an optical path length measured by a distance measuring interferometer into a geometric length.
Such distance, angle, and dispersion interferometers are crucial components of scanner systems and stepper systems used in lithography to produce integrated circuits on semiconductor wafers. The lithography systems typically include: at least one movable stage to support, orient, and fix the wafer; focusing optics used to direct a radiation beam onto the wafer; a scanner or stepper system for translating the stage relative to the exposure beam; and one or more interferometers to accurately measure changes in the position of the stage relative to the radiation beam. The interferometers enable the lithography system to precisely control which regions of the wafer are exposed to the radiation beam.
The invention features an interferometry system which includes at least one dynamic beam steering assembly for redirecting one or more beams within the interferometry system in response to changes in the angular orientation or position of the measurement object. The dynamic beam steering assembly can be incorporated into interferometry systems that measure distances, angle, and/or dispersion. The dynamic beam steering assembly improves the performance of the interferometry system by minimizing negative consequences of changes in the angular orientation or position of the measurement object.
In general, in one aspect, the invention features an interferometry system including: an interferometer which during operation directs a measurement beam along a measurement path contacting a measurement object and combines at least a portion of the measurement beam with another beam to form an overlapping pair of exit beams, the interferometer including a beam steering assembly having a beam steering element and a positioning system to orient the beam steering element, the beam steering element positioned to direct the measurement beam, the measurement beam contacting the beam steering element; and a control circuit which during operation causes the positioning system to reorient the beam steering element in response to a change in at least one of angular orientation and position of the measurement object. The interferometer can direct the measurement beam to contact the measurement object a single time (single-pass), two times (double pass), or an even or odd numbers of times.
During operation the control circuit can cause the positioning system to reorient the beam steering element in response to a change in angular orientation of the measurement object. In one such embodiment, the interferometry system can further include a signal processor. During operation the interferometer directs a second measurement beam along a second measurement path contacting the measurement object and combines at least a portion of the second measurement beam with an additional beam to form a second overlapping pair of exit beams. The measurement beams contact the measurement object at separate locations, and during operation the signal processor determines the change in angular orientation of the measurement object based on signals derived from the overlapping pairs of exit beams. In the latter embodiment, the beam steering element can be positioned to direct both of the measurement beams, both of the measurement beams contacting the beam steering element.
In general, in another aspect, the invention features an interferometry system including: an interferometer which during operation receives an input beam, splits the input beam into a measurement beam and at least one other beam, directs the measurement beam along a measurement path contacting a measurement object, and combines at least a portion of the measurement beam with the other beam to form an overlapping pair of exit beams; a beam steering assembly having a beam steering element and a positioning system to orient the beam steering element, the beam steering element positioned to direct the input beam and the overlapping pair of exit beams, the input beam and the overlapping pair of exit beams contacting the beam steering element; a control circuit which during operation causes the positioning system to reorient the beam steering element in response to a change in at least one of angular orientation and position of the measurement object. The interferometer can direct the measurement beam to contact the measurement object a single time (single-pass), two times (double pass), or an even or odd numbers of times.
During operation the control circuit can cause the positioning system to reorient the beam steering element in response to a change in angular orientation of the measurement object. In one such embodiment, the interferometry system can further include a signal processor. During operation the interferometer directs a second measurement beam along a second measurement path contacting the measurement object and combines at least a portion of the second measurement beam with an additional beam to form a second overlapping pair of exit beams. The measurement beams contact the measurement object at separate locations, and during operation the signal processor determines the change in angular orientation of the measurement object based on signals derived from the overlapping pairs of exit beams. In the latter embodiment, the beam steering element can be positioned to direct the input beam and both of the overlapping pairs of exit beams, all of which contact the beam steering element.
In either of the interferometry systems described above, the system can further include a signal processor and the measurement beam can include components at two wavelengths. In one such embodiment, the interferometer produces the first mentioned overlapping pair of exit beams and a second overlapping pair of exit beams. The first pair of exit beams is indicative of changes in optical path length to the measurement object at a first of the two wavelengths, and the second pair of exit beams is indicative of changes in optical path length to the measurement object at a second of the two wavelengths. During operation the signal processor processes signals derived from the overlapping pairs of exit beams. For example, the signal processor can calculate dispersion along the measurement path based on the signals, and/or the signal processor can calculate changes in geometric distance to the measurement object based on the signals. The two wavelengths are sufficiently separated to permit such calculations, e.g., they are harmonically related or are separated by at least one 1 nm.
In another such embodiment, the interferometry system further includes the signal processor, and the measurement beam, the other beam, and the overlapping pair of exit beams each include components at the two wavelengths. The overlapping pair of exit beams is indicative of changes in optical path length to the measurement object at each of the two wavelengths, and during operation the signal processor processes signals derived from the overlapping pair of exit beams. For example, the signal processor can calculate dispersion along the measurement path based on the signals, and/or the signal processor can calculate changes in geometric distance to the measurement object based on the signals. Again, the two wavelengths are sufficiently separated to permit such calculations, e.g., they are harmonically related or are separated by at least one 1 nm.
In general, in another aspect, the invention features an interferometry method including: directing a measurement beam along a measurement path contacting a measurement object; combining at least a portion of the measurement beam with another beam to form an overlapping pair of exit beams; and using an electronic control system to redirect the measurement beam in response to a change in at least one of the angular orientation and position of the measurement object. The measurement path can contact the measurement object one time, two times, or more generally, an even or odd number of times. The method can further include: directing a second measurement beam along a second measurement path contacting the measurement object; combining at least a portion of the second measurement beam with an additional beam to form a second overlapping pair of exit beams, the first and second pairs of exit beams indicative of changes in optical path length to two separate locations on the measurement object; and calculating the change in angular orientation of the measurement object based on signals derived from the overlapping pairs of exit beams.
In general, in another aspect, the invention features an interferometry method including: directing a measurement beam having components at two wavelengths along a measurement path contacting a measurement object; calculating changes in geometric distance to the measurement object based on interferometric signals derived from the measurement beam; and using an electronic control system to redirect the measurement beam in response to a change in at least one of the angular orientation and position of the measurement object. Alternatively, in another aspect, the invention features an interferometry method including: directing a measurement beam having components at two wavelengths along a measurement path contacting a measurement object; calculating dispersion along the measurement path based on interferometric signals derived from the measurement beam; and using an electronic control system to redirect the measurement beam in response to a change in at least one of the angular orientation and position of the measurement object. The electronic control system in either of these aspects can redirect the measurement beam in response to a change in the angular orientation of the measurement object, and in particular, can redirect the measurement beam to contact the measurement object at substantially normal incidence over a range of angular orientations of the measurement object. Also, in either of these aspects, the two wavelengths are sufficiently separated to permit the geometric length or dispersion calculation, e.g., they are harmonically related or are separated by at least one 1 nm.
The invention also features a lithography system for fabricating integrated circuits including first and second components, which are movable relative to one another. One of interferometry systems described above is secured to the second component and the measurement object is a mirror rigidly secured to the first component. During operation the interferometry system measures the position of the first component relative to the second component. In some embodiments, the second component is a movable stage used to support a wafer and during operation the beam steering element causes the measurement beam to contact the mirror at substantially normal incidence over a range of angular orientations of the measurement object.
The interferometry systems and methods described above can further include one or more features of any of the following additional aspects of the invention.
In general, in another aspect, the invention features an interferometry system for measuring changes in distance to a measurement object. The system includes an interferometer which during operation directs a reference beam along a reference path and a measurement beam along a measurement path contacting the measurement object and combines the reference and measurement beams to produce overlapping exit reference and measurement beams. The overlapping exit reference and measurement beams are indicative of changes in a relative optical path length between the reference and measurement paths. The interferometer includes a beam steering assembly having a beam steering element and a positioning system to orient the beam steering element. The beam steering element is positioned to direct at least one of the reference and measurement beams and the at least one of the reference and measurement beams contact the beam steering element. The system also includes a control circuit which during operation causes the positioning system to reorient the beam steering element in response to changes in at least one of angular orientation and position of the measurement object.
The system can include any of the following features.
The interferometer can separate an input beam into a pair of spatially separated beams to define the reference and measurement beams. Alternatively, during operation the interferometer receives a pair of spatially separated input beams to define the reference and measurement beams.
The interferometer can be a single-pass interferometer in which the measurement beam contacts the measurement object only a single time. Also, in some embodiments, the interferometer can include the measurement object, and in other embodiments, the measurement object can be separate from the interferometer. During operation the control circuit can cause the positioning system to reorient the beam steering element based on a signal generated from the exit measurement beam.
Also, during operation, the control circuit can cause the positioning system to reorient the beam steering element to do any of the following: 1) maintain the exit reference and measurement beams substantially parallel to one another over a range of angular orientations of the measurement object;. 2) reduce changes in direction of the exit measurement beam caused by changes in the angular orientation of the measurement object; 3) reduce a transverse displacement between the exit reference and measurement beams caused by changes in the angular orientation of the measurement object or a translation of the measurement object; 4) reduce a transverse displacement of the measurement beam caused by changes in the angular orientation of the measurement object or a translation of the measurement object; and 5) insure that the measurement beam contacts the measurement object at substantially normal incidence over a range of angular orientations of the measurement object.
In embodiments in which the measurement object is a plane mirror and during operation the measurement beam contacts the measurement object at non-normal incidence, the control circuit can cause the positioning system to reorient the beam steering element to reduce transverse displacements between the exit reference and measurement beams caused by by translations of the measurement object.
In some embodiments, the beam steering element can include a polarizing beam splitter that separates the input beam into the reference and measurement beams, and the positioning system can include at least one transducer mounted to the polarizing beam splitter and operative to adjust an orientation of the polarizing beam splitter in response to a control signal from the control circuit. Alternatively, the beam steering element can include a polarizing beam splitter that directs the reference beam along the reference path and the measurement beam along the measurement path, and the positioning system can include at least one transducer mounted to the polarizing beam splitter and operative to adjust an orientation of the polarizing beam splitter in response to a control signal from the control circuit.
In other embodiments, the interferometer can further include a polarizing beam splitter that separates the input beam into the reference and measurement beams, and wherein the beam steering element includes a beam steering mirror and the positioning system includes at least one transducer mounted to the beam steering mirror and operative to adjust an orientation of the beam steering mirror in response to a control signal from the control circuit. Alternatively, the interferometer can further include a polarizing beam splitter that directs the reference beam along the reference path and the measurement beam along the measurement path, and wherein the beam steering element includes a beam steering mirror and the positioning system includes at least one transducer mounted to the beam steering mirror and operative to adjust an orientation of the beam steering mirror in response to a control signal from the control circuit.
The system can further include a polarizer which during operation receives the exit reference and measurement beams and mixes polarizations of the exit reference and measurement beams to produce a mixed beam having a phase indicative of the changes in the optical path length difference between the reference and measurement paths. The control circuit can include a detector having spatially resolved detector elements operative to measure directions and/or positions of the exit reference and measurement beams and generate a measurement signal indicative of the directions and/or positions. The control circuit can further include a controller operative to receive the measurement signal from the detector and send a control signal based on the measurement signal to the positioning system, the control signal causing the positioning system to reorient the beam steering element.
In addition to a polarizing beam splitter, beam steering mirror, and transducers mounted to the beam steering mirror, the interferometer can further include a measurement retroreflector, a reference retroreflector, and a reflective reference object. During operation, the polarizing beam splitter directs the reference beam to the reference object and the measurement beam to the measurement retroreflector, receives the reference beam from the reference object and the measurement beam from the measurement retroreflector, and then directs the reference and measurement beams to the beam steering mirror. The beam steering mirror directs the reference beam to the reference retroreflector and the measurement beam to the measurement object, receives the reference beam from the reference retroreflector and the measurement beam from the measurement object, and then directs the reference and measurement beams back to the polarizing beam splitter. The polarizing beam splitter combines the reference and measurement beams to produce the overlapping exit reference and measurement beams. The measurement retroreflector can have an aperture through which the reference beam travels to and from the reference object and the reference retroreflector can have an aperture through which the measurement beam travels to and from the measurement object. The interferometer can further include a reference quarter wave plate through which the reference beam travels to and from the reference object and a measurement quarter wave plate through which the measurement beam travels to and from the measurement object.
The beam steering mirror can have front and back faces. During operation the front face of the beam steering mirror can direct the measurement beam to the measurement object and the back face of the beam steering mirror can receive the measurement beam from the measurement object. The interferometer can further include a second polarizing beam splitter which during operation receives the reference beam from the reference path and the measurement beam from the back face of the beam steering mirror and produces the exit reference and measurement beams.
The interferometer can further include at least one additional beam steering assembly including an additional beam steering element and an additional positioning system to orient the additional beam steering element. During operation the additional beam steering element directs the measurement beam. The measurement beam contacts the additional beam steering element and during operation the control circuit causes the positioning systems to reorient the beam steering elements to optimize the overlap of the exit reference and measurement beams and maintain the exit reference and measurement beams substantially parallel over a range of angular orientations and positions of the measurement object. The control circuit can include two detectors having spatially resolved detector elements operative to measure a position and direction of the exit measurement beam and generate measurement signals indicative of the position and direction. The control circuit can further include a controller operative to receive the measurement signals from the detectors and send control signals to the positioning systems based on the signals from the detectors, the control signals causing the positioning systems to reorient the beam steering elements.
In general, in another aspect, the invention features an interferometry system for measuring changes in distance to a measurement object. The system includes an interferometer which during operation receives one or more input beams to define a reference beam and a measurement beam, directs the reference beam along a reference path and the measurement beam along a measurement path contacting the measurement object, and combines the reference and measurement beams to produce an output beam including overlapping exit reference and measurement beams. The output beam is indicative of changes in a relative optical path length between the reference and measurement paths. The system further includes a beam steering assembly including a beam steering element and a positioning system to orient the beam steering element. During operation the beam steering element directs the input and output beams, the input and output beams contacting the beam steering element. The system further includes a control circuit which during operation causes the positioning system to reorient the beam steering element in response to changes in at least one of angular orientation and position of the measurement object.
The system can include any of the following features.
The interferometer can receive a single input beam and separate the single input beam into a pair of spatially separated beams to define the reference and measurement beams. Alternatively, the interferometer can receive a pair of spatially separated input beams to define the reference and measurement beams.
The beam steering element can include a beam steering mirror, and the positioning system can include at least one transducer mounted to the beam steering mirror and operative to adjust an orientation of the beam steering mirror in response to a control signal from the control circuit. The beam steering mirror can include front and back faces, and wherein during operation the input beam contacts the front face of the beam steering mirror and the output beam contacts the back face of the beam steering mirror. The interferometer can be a single-pass interferometer in which the measurement beam contacts the measurement object only a single time.
The control circuit can cause the positioning system to reorient the beam steering element to do any of the following: 1) maintain the exit reference and measurement beams substantially parallel to one another over a range of angular orientations of the measurement object; 2) reduce changes in direction of the exit measurement beam caused by changes in the angular orientation of the measurement object; 3) to reduce a transverse displacement between the exit reference and measurement beams caused by changes in angular orientation of the measurement object or a translation of the measurement object; 4) reduce a transverse displacement of the measurement beam caused by changes in angular orientation of the measurement object or a translation of the measurement object; 5) to insure that the measurement beam contacts the measurement object at substantially normal incidence over a range of angular orientations of the measurement object.
In general, in a further aspect, the invention features an interferometry system including an interferometer which during operation directs a reference beam along a reference path and a measurement beam along a measurement path contacting a measurement object at non-normal incidence, and combines the reference and measurement beams to form overlapping exit reference and measurement beams. The overlapping exit measurement and reference beams are indicative of changes in a relative optical path length between the reference and measurement paths. The interferometer includes a beam steering assembly positioned to direct the measurement beam. The system includes a control circuit which during operation causes the beam steering assembly to redirect the measurement beam to reduce transverse displacements between the exit reference and measurement beams caused by translations of the measurement object.
In general, in another aspect, the invention features an interferometry system including an interferometer which during operation directs a reference beam along a reference path and a measurement beam along a measurement path contacting a measurement object, and combines the reference and measurement beams to form overlapping exit reference and measurement beams. The overlapping exit measurement and reference beams are indicative of changes in a relative optical path length between the reference and measurement paths. The interferometer includes a beam steering assembly positioned to direct at least one of the reference and measurement beams.
The system further includes a control circuit which during operation causes the beam steering assembly to redirect one of the reference and measurement beams to do at least one of the following: 1) maintain the exit reference and measurement beams substantially parallel to one another over a range of angular orientations of the measurement object; 2) reduce changes in direction of the exit measurement beam caused by changes in the angular orientation of the measurement object; 3) reduce a transverse displacement between the exit reference and measurement beams caused by changes in angular orientation of the measurement object; 4) reduce a transverse displacement between the exit reference and measurement beams caused by translations of the measurement object; 5) to insure that the measurement beam contacts the measurement object at substantially normal incidence over a range of angular orientations of the measurement object; 6) to reduce a transverse displacement of the measurement beam caused by translations of the measurement object; and 7) to reduce a changes in angular orientation of the measurement object.
In general, in another aspect, the invention features an interferometry system including an interferometer which during operation directs a reference beam along a reference path and a measurement beam along a measurement path contacting a measurement object, and combines the reference and measurement beams to form overlapping exit reference and measurement beams. The overlapping exit measurement and reference beams are indicative of changes in a relative optical path length between the reference and measurement paths. The interferometer includes a beam steering assembly positioned to direct at least one of the reference and measurement beams. The system further includes a control circuit which during operation causes the beam steering assembly to redirect the at least one of the reference and measurement beams in response to changes in angular orientation of the measurement object based on a signal generated from the exit measurement beam.
In general, in another aspect, the invention features a single-pass interferometry system including an interferometer which during operation directs a reference beam along a reference path and a measurement beam along a measurement path contacting a measurement object, and combines the reference and measurement beams to form overlapping exit reference and measurement beams. The overlapping exit measurement and reference beams are indicative of changes in a relative optical path length between the reference and measurement paths. The measurement path contacts the measurement object only a single time. The interferometer includes a beam steering assembly positioned direct at least one of the reference and measurement beams. The system further includes a control circuit which during operation causes the beam steering assembly to redirect the at least one of the reference and measurement beams in response to changes in angular orientation of the measurement object.
In another aspect, the invention also features a lithography system for use in fabricating integrated circuits on a wafer. The lithography system includes: a stage for supporting the wafer; an illumination system for imaging spatially patterned radiation onto the wafer; a positioning system for adjusting the position of the stage relative to the imaged radiation; and at least one of any of the interferometry systems described above. The interferometry system(s) measures the position of the wafer relative to the imaged radiation.
In another aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer. The lithography system includes a stage for supporting the wafer and an illumination system. The illumination system includes a radiation source, a mask, a positioning system, a lens assembly, and at least one of any of the interferometry systems described above. During operation the source directs radiation through the mask to produce spatially patterned radiation. The positioning system adjusts the position of the mask relative to the radiation from the source. The lens assembly images the spatially patterned radiation onto the wafer. The interferometry system(s) measures the position of the mask relative to the radiation from the source.
In another aspect, the invention features a lithography system for fabricating integrated circuits. The lithography system includes first and second components, the first and second components being movable relative to each other. The lithography system also includes at least one of any of the interferometry systems described above, wherein the first component includes the measurement object and the interferometry system(s) monitors the position of the first component relative to the second component.
In another aspect, the invention features a beam writing system for use in fabricating a lithography mask. The beam writing system includes: a source providing a write beam to pattern a substrate; a stage supporting the substrate; a beam directing assembly for delivering the write beam to the substrate; a positioning system for positioning the stage and beam directing assembly relative one another; and at least one of any of the inteferometry systems described above for measuring the position of the stage relative to the beam directing assembly.
In general, in another aspect, the invention features a method for interferometry. The interferometry method includes: directing a reference beam along a reference path and a measurement beam along a measurement path contacting a measurement object; combining the reference and measurement beams to form overlapping exit reference and measurement beams, the overlapping exit measurement and reference beams indicative of changes in a relative optical path length between the reference and measurement paths; and using an electronic control system to redirect the measurement beam in response to changes in at least one of angular orientation and position of the measurement object.
In general, in another aspect, the invention features a method for interferometry. The interferometry method includes: receiving one or more input beams to define a reference beam and a measurement beam; directing the reference beam along a reference path and the measurement beam along a measurement path contacting a measurement object; combining the reference and measurement beams to form an output beam including overlapping exit reference and measurement beams, the overlapping exit measurement and reference beams indicative of changes in a relative optical path length between the reference and measurement paths; and using an electronic control system to redirect the input and output beams in response to changes in at least one of angular orientation and position of the measurement object.
In another aspect, the invention features a lithography method for use in fabricating integrated circuits. The lithography method includes: imaging spatially patterned radiation onto a wafer; positioning the wafer relative to the imaged radiation; and measuring the position of the wafer relative to the imaged radiation using at least one of the interferometry methods described above.
In another aspect, the invention features a lithography method for use in the fabrication of integrated circuits. The lithography method includes: directing input radiation through a mask to produce spatially patterned radiation; positioning the mask relative to the input radiation; measuring the position of the mask relative to the input radiation using at least one of the interferometry methods described above; and imaging the spatially patterned radiation onto a wafer.
In another aspect, the invention features a lithography method for use in fabricating integrated chips. The lithography method includes: positioning a first component of a lithography system relative to a second component of a lithography system to expose a wafer to spatially patterned radiation; and measuring the position of the first component relative to the second component using any of the interferometry methods described above.
In another aspect, the invention features a beam writing method for use in fabricating a lithography mask. The method includes: directing a write beam to a substrate to pattern the susbtrate; positioning the substrate relative to the write beam; and measuring the position of the substrate relative to the write beam using any of the interferometry methods described above.
Finally, in general, in another aspect, the invention features an interferometry system for measuring changes in distance to a measurement object. The interferometry system includes an interferometer, a beam steering assembly for redirecting at least one beam within the interferometer, and a control circuit for reorienting the beam steering assembly in response to changes in the angular orientation or position of the measurement object.
Embodiments of the interferometry systems and methods described above include many advantages.
For example, the system can maintain the exit reference and measurement beams substantially parallel to one another over a range of orientations of the measurement object, and can do so with only a single pass of the measurement beam to the measurement object. The single-pass system reduces the bandwidth of electronics needed to process electrical interference signals having Doppler shifts, relative to those of a double-pass interferometer. Furthermore, the single-pass system reduces the likelihood of depolarization, scattering, and undesired spurious reflections from transmissive optics within the interferometer, relative to those for a double-pass interferometer. Such effects can introduce errors, e.g., cyclic errors, in the measured phase of the electrical interference signal. Nonetheless, embodiments can also include multi-pass, e.g., double-pass, configurations.
In both single-pass and multi-pass configurations, the system can minimize the transverse displacement of the exit reference and measurement beams or the transverse displacement of components of the reference and measurement beams within the interferometer caused by changes in the angular orientation or position of the measurement object. As a result, the average amplitude of the electrical interference signal produced from the mixed exit reference and measurement beams can be substantially independent of changes in the angular orientation and position of the measurement object. In addition, the system reduces changes of the paths of the measurement and reference beams through transmissive optics of the interferometer, e.g., polarizing beam splitters and quarter wave plates, caused by changes in the angular orientation or position of the measurement object. Such transmissive optics can have imperfections in their surface figures and local variations in refractive index. Thus, changes in the paths of the beams through such optics can change the optical path length measured by the interferometer even though the distance between the measurement object and the interferometer has not changed. Such negative effects can be compounded if the transmissive optic has dispersive properties, such as those produced by a wedge.
Furthermore, in some embodiments, the system includes one or more polarizing beam splitters as the only transmissive optics, with all other optics being reflective. In particular, quarter wave plates are not required. Minimizing the number of transmissive optics in the system optics reduces depolarization, scattering, and undesired spurious reflections of the reference and measurement beams, which can introduce errors, e.g., cyclic errors, in the measured phase of the electrical interference signal.
The interferometry systems can also include multiple interferometers and at least one beam steering assembly to measure angular orientation, in addition to displacement, of the measurement object. Such angle-measuring embodiments can enjoy many of the advantages described above.
Furthermore, the interferometer systems can involve beam components at multiple wavelengths to measure dispersion along the measurements path(s). Such dispersion measurements can be used to convert optical path length changes into geometric path length changes. Again such dispersion-measuring embodiments can enjoy many of the advantages described above.
Similar features are also described in commonly owned, copending U.S. patent application Ser. No. 09/157,131 entitled xe2x80x9cInterferometer Having a Dynamic Beam Steering Assemblyxe2x80x9d by Henry A. Hill and Peter de Groot filed Sep. 18, 1998, the contents of which are incorporated herein by reference.
Other features, aspects, and advantages will be apparent from the following detailed description and from the claims.